By Danielle Moloney
While fall makes its way into the Northern Hemisphere, bringing changing leaves and cooling temperatures, waters remain warm in the tropics. As a result, hurricanes continue to dominate news headlines for the terrestrial, social, and economic devastation that they leave in their wakes. NOAA recognizes hurricane season in the Atlantic from June until November, typically peaking in September for the eastern Caribbean. While much scientific research is dedicated to understanding hurricanes’ destruction of terrestrial environments, which tends to have more visibility and immediate human impact, it is also worthwhile to study the impact that hurricanes and other natural disasters have under the waves. That is what a recent study from Dr. Caroline S. Rogers at the Wetland and Aquatic Research Center in St. John, US Virgin Islands (USVI), sought to do by analyzing the effect of the 2017 hurricane season on local coral reefs and mangrove forests. The study was published in the journal Diversity in August 2019.
Each hurricane season, meteorologists assign categories to approaching hurricanes based on their predicted level of damage caused to man-made and natural structures. Category 1 is the least dangerous, with wind speeds below 100 mph and some damage to power lines and homes. Category 5, the most severe, predicts winds of 157 mph and higher, weeks- to months-longmonths long power outages, and destruction of homes. Category 5 hurricanes batter coastal communities across the Caribbean and into the United States each hurricane season. Recent years have brought about more frequent hurricanes and severe storm seasons, which have been linked to rising global temperatures. Warmer temperatures cause higher seas and atmospheric moisture levels, both of which contribute to the overall intensity of storms. These factors lead to a multitude of concerns for communities in the path of hurricanes: higher storm surges, increased rainfall and flooding, and stronger winds than average hurricanes in the past . All of these circumstances spell catastrophe for terrestrial systems and humanity –but what do they mean for coral reefs and mangroves?
Reefs and Mangroves in the USVI
In September 2017, two Category 5 hurricanes, Maria and Irma, slammed into the Caribbean Islands less than two weeks apart. They are remembered for their unprecedented widespread destruction, the effects of which are still experienced today. Dr. Rogers studied the effects of Hurricane Irma at the Virgin Islands Coral Reef National Monument in St. John. Though researchers could not quantify the underwater damage immediately after the storms due to dangerous ocean conditions, in February 2018, five months after Maria and Irma passed over, they found evident disruption to both reef and mangrove systems (Figure 1).
The mangroves of St. John provide an uncommon link between the terrestrial and marine environments on the island, and researchers have found that coral species have been demonstrated to use the underwater roots of the mangroves as a building substrate. Previous studies suggest that shade from mangrove trees keeps water and light levels in a preferable range for coral survival, and even increases survival in study sites where bleaching and mortality havehas been observed in the same species of coral that is not growing under the shade of a tree. Waves and debris from the hurricanes removed most large corals and sponges in the mangroves and stripped the mangrove roots (Figure 2). Furthermore, above-ground destruction of mangrove trees decreased the shade in the Water Creek study site, effectively removing the variable water temperatures and low light levels in the bay which previously allowed corals to thrive there.
While there is more research that must be done to fully understand what recovery in this area will look like, Rogers expects that post-storm recovery will be slow-moving, as both corals and mangroves grow slowly. She notes that community composition will likely be permanently altered by destruction from the storms, as smaller corals will stand a better chance at survival (small corals are less likely to be pulverized by debris than branching corals and tend to have a faster growth rate than large corals) whereas larger, ‘builder’ and branching coral species will continuously be smashed by high-intensity storms each hurricane season. Scientists like Rogers cannot yet predict the future of St. Johns reefs yet, because stress from natural disasters is markedly different from other stressors that plagque reefs, including warm ocean temperatures and diseases.
Though hurricanes are a natural part of weather systems, scientists are concerned by the predictedby predicted rise in their frequency and intensity, coupled with other mounting threats to coral reefs. Though the nature of post-hurricane marine environments makes immediate effects difficult to study due to hazardous waters with debris and high waves, valuable information can be obtained from longitudinal studies that track reef recovery and resilience in the months and years after natural disasters strike.
Some optimistic outlooks for corals have been observed when hurricanes occur. Partially destroyed corals can sometimes settle elsewhere and create new reefs. The destruction of one coral species can lend itself to the success of another species. Though controversial, land structures that are destroyed and fall into the ocean can serve as artificial substrates for new reefs to grow from.
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